1. Field of the Invention
The present invention relates to an optical transmission device. In particular, the present invention relates to the following: an optical element that modulates output light of a semiconductor laser from outside; or an optical transmission device that has an output portion for coupling an output of this optical element to a transmission optical fiber; or a novel structure of a semiconductor optical detection device. The present invention is particularly useful as an optical transmitter-receiver that is suitable for a high-speed optical communication system capable of handling 10 gigabit per second or more.
2. Related Art
Optical modulators and photodetectors, which are used for optical communication, are broadly classified into a surface incidence type, a waveguide type, a traveling wave type based on element structure. Among them, elements of the surface incidence type and the waveguide type have a limit of operation speed resulting from the so-called CR time constant (element capacity C, element resistance R). Therefore, in order to realize speedup, it is necessary to reduce C by reducing an area of the element, or by other means.
On the other hand, the traveling-wave type element is devised as an improved version of the waveguide-type element. An electrode, which is formed in proximity to an optical waveguide, is treated as a transmission line. A high-frequency electric signal propagates through the electrode in the form of a traveling wave. In this case, it is possible to realize high-speed operation, which does not depend on the CR time constant, by controlling traveling speed Vopt of a light wave and propagation velocity Vele of a high-frequency electric signal so that they become equal as close as possible. In general, as main waveguide materials for a traveling-wave type element, the following are under examination: niobic acid lithium for optical modulator use, and an optical modulator; and compound semiconductors for photodetector use such as an InP group and a GaAs group. Among them, as regards the traveling-wave element in which a compound semiconductor is used, mainly because difference in refractive indexes (relative dielectric constant) between an electric signal and a light signal is large, it is generally difficult to make the relation between Vopt and Vele equal. If a compound semiconductor is used, there is a high possibility that monolithic integration with other optical parts such as a light source, and miniaturization of an element resulting from this, can be realized. For the purpose of solving the above-mentioned mismatch of speed between an optical signal and an electric signal, which relates to the traveling-wave element that uses a compound semiconductor, some techniques has been devised.
As one of the conventional improvement techniques, the following example is reported: at the top of an optical waveguide made on a semiconductor substrate, or in close proximity to the top of the optical waveguide, a traveling-wave electrode is integrally formed; and high-frequency characteristics have been improved particularly by making a shape of a traveling-wave type electrode appropriate. In this configuration, optimum design of a semiconductor structure and optimum design of a traveling-wave electrode should be performed in the same element. In addition, if in particular a length of a waveguide in a light-wave travelling direction becomes about 1000 μm or more, it is necessary to make a very thick metal electrode having a thickness of 4 through 10 μm, for example, on a semiconductor element in order to improve attenuation of intensity resulting from traveling of a high-frequency electric signal. This electrode is thicker than an electrode of a general semiconductor optical element by a factor of about ten. Therefore, there is a large mismatch of structure between a general optical semiconductor element and a traveling wave electrode.
In addition, as another improvement technique, a technique by which phase matching of an electromagnetic wave and a light wave is improved by separating an electrode on the upper part of a waveguide cyclically has been proposed. Although pseudo phase matching is achieved by means of this technique, an element size is large, and an element manufacturing process is also very complicated.
Because of the background as described above, the traveling-wave optical elements, which use a semiconductor, have not been broadly put to practical use under the existing circumstances.
In this connection, as documents that describe these conventional semiconductor traveling-wave electrode optical element, the following can be named: IEE Electronics Letters, the 18th issue, vol. 33, page 1580, Aug. 28, 1997; and International Conference on Indium Phosphorus, paper number WeA1-3, preprint page 385, 1998.
On the other hand, as a new document relating to the present invention, Japanese Patent Application Laid-Open No. Hei 6-160788 can be named. In this document, for the purpose of realizing a niobic acid lithium optical modulator, a driving voltage of which is low, and an optical insertion loss of which is also low, and for the purpose of reducing a propagation loss of a high-frequency electric signal, an introduction of an oxide superconduction electrode is proposed. However, under the existing circumstances, stable operation of oxide superconduction electrode materials at room temperature is not realized. Therefore, this method has not been put to practical use.
A main object of the present invention is to realize an economical optical transmitter-receiver having a simple structure, which is suitable for a high-speed optical communication system. Another object of the present invention is to improve operation speed of a traveling-wave element having a compound semiconductor, which is particularly suitable for monolithic accumulation with other optical parts such as a semiconductor laser, by means of an easy technique.